The present invention relates to programmable latches that include non-volatile programmable elements. Examples of non-volatile programmable elements are fuses.
Programmable latches with fuses are used in integrated circuits to enable modification of the circuits without changing the masks used for circuit fabrication. For example, fuse programmable latches are used to replace defective memory cells with spare cells in semiconductor memories.
FIG. 1 shows one such latch described in U.S. Pat. No. 4,546,455 issued Oct. 8, 1985 to Iwahashi et al. Fuse F1 is connected between the power supply terminal VDD and the latch output terminal OUT. An NMOS transistor 110 is connected between the terminal OUT and the ground terminal. Three serially connected inverters 120.1, 120.2, 120.3 are provided between the output terminal OUT and the gate of transistor 110.
If fuse F1 is intact, the terminal OUT is pulled to VDD. The inverters 120 keep transistor 110 off. If fuse F1 is blown, the terminal OUT is at the ground voltage. The inverters turn transistor 110 on to keep the terminal OUT at ground.
Bypass capacitor 130 is connected between the input of inverter 120.3 and the ground terminal to insure correct initialization of the latch on power-up. More particularly, when the power is off, transistor 110 is non-conductive. Therefore, terminal OUT is floating. According to U.S. Pat. No. 4,546,455, the terminal OUT may "tend to have its potential raised to a logic level `1`". In addition, the terminal OUT potential may become "unstable due to the capacitive coupling" in the integrated circuit. Consequently, during operation, transistor 110 could be off even if the fuse F1 were blown. As a result, the terminal OUT would provide an erroneous voltage.
Capacitor 130 is intended to avoid such malfunction. During power up, capacitor 130 keeps the input of inverter 120.3 low sufficiently long to allow the inverter to turn on transistor 110 and discharge the terminal OUT to ground if the fuse is blown. Then the ground voltage on terminal OUT propagates through the three inverters to keep transistor 110 on.
It is desirable to provide alternative programmable latches.